This invention relates generally to structures for interconnecting integrated circuits disposed on opposite sides of an insulating substrate. More specifically, the present invention relates to methods and apparatus for enabling electronic components mounted on both sides of an insulating substrate to communicate electrically without having to remove areas of the substrate to form a via.
A main focus of the contemporary semiconductor industry is miniaturization, which is furthered by increasing the density at which integrated circuits are mounted on a substrate, such as a printed circuit board. Miniaturization enables the design and manufacture of increasingly smaller and more compact devices such as hand-held computers, personal data assistants (PDA) and portable telecommunications devices.
One way to satisfy the demand for increased integrated circuit density is to attach integrated circuits to opposing sides of a dielectric circuit board. A technique developed by the semiconductor industry to couple integrated circuits positioned on opposite sides of a circuit board to each other has been the creation of conductive vias through circuit boards. FIG. 1 is a partial isometric view of a circuit board 1 comprising a substrate 8 with holes 10 extending through its thickness 11 in any desired arrangement according to the prior art. The substrate 8 may comprise a BT or FR dielectric core, and the methods used to make the holes 10 include drilling, etching and laser ablation.
The circuit board 1 is shown in greater detail in FIG. 2, which is a partial cross-sectional view of the circuit board 1 at a later stage of production. After the holes 10 have been formed in the substrate 8, they are plated to form conductive linings 12. The conductive linings 12 extend from the first surface 15 of the substrate 8 to the second surface 16 of the substrate 8. The lining 12 is generally comprised of copper, but any other conductive material may be used. Conductive traces 13 are also deposited on the first and second surfaces 15 and 16 of the substrate 8. The conductive traces 13 couple integrated circuits to other integrated circuits mounted on the same surface 15, 16. Additionally, the conductive traces 13 couple integrated circuits to the linings 12.
A solder mask 14 may then be applied to both the first surface 15 and the second surface 16 to insulate conductive traces 13 and conductive via linings 12, and to protect them from deleterious environmental factors such as dust or moisture. Areas of the traces 13 and linings 12 to which conductive integrated circuit leads will be soldered are left uncoated by the solder mask 14. Integrated circuits (not shown) are then soldered to the traces 13 on the first surface 15 and the second surface 16.
There are several problems associated with this type of prior art technique however, with perhaps the most significant being the limited density at which vias may be formed. Each via must inherently have a diameter 18 greater than the diameter 17 of its hole 10 to enable traces 13 to connect to the via. This outer diameter 18 of the contact surface, or contact pad 19, typically amounts to twice the diameter 17 of the hole 10. Therefore, even if the diameter of the holes 10 could be decreased, the density of the vias would still be limited by the outer diameter 18 of the contact pad 19.
Presently, holes 10 can be drilled with diameters as small as 50 xcexcm. This limitation arises due to difficulties in forming narrow holes 10 in the substrate 8, as well as difficulties in plating the inner surfaces of the holes 10. As a consequence, after the addition of the necessary capture pad, the prior art cannot create vias with diameters of less than 100 xcexcm. This significantly constrains efforts to increase circuit board density, and prevents the development of smaller and more compact electronic devices.
What is needed is a method of electrically coupling two integrated circuits on opposite sides of a circuit board without the prior art solution of creating and filling holes in the circuit board.
The present invention is directed to an apparatus and method for connecting integrated circuits placed on opposite sides of a circuit board through utilization of conduction elements embedded in the circuit board. The conduction elements extend from one surface of the circuit board to the other. Conductive traces coupled to integrated circuits make contact with the conduction elements to allow integrated circuits mounted on opposite surfaces of the board to be coupled to each other.